D800 How User Friendly Is The Viewfinder In DX Mode?

With respect to the D800 viewfinder image in DX crop mode, could I ask how easy/difficult you think it will be/find it, to achieve accurate manual focus with the reduced viewfinder image size when using crop mode, or, is the "masked" area in viewfinder image still clear enough to "see through" to help achieve manual focus?

I am assuming that the crop modes do in fact mask out the area around the crop image in the viewfinder. It seems there might be some doubt about that actually being the case.

I am concerned that the viewfinder image size in crop mode will be too small to achieve accurate manual focus, especially when using long telephoto lenses.

This could be a the deciding factor as to whether I opt for the D800.

I appreciate any guidance and advice.